The present invention relates generally to threaded fasteners, and, more particularly, to female threaded fasteners that are self-locking.
Threaded fasteners of this particular kind are useful in a wide variety of applications, where load bearing conditions can vary significantly and where high vibrations can otherwise act to loosen or release fasteners that are not self-locking. Most conventional self-locking fasteners require the application of a substantial torque throughout substantially the entire range of threaded engagement with a bolt. This characteristic presents a number of disadvantages, including increased wear on the fastener threads and on the driving surfaces of the bolt or fastener, increased operator fatigue, and increased assembly time.
One solution to some of the disadvantages of conventional self-locking fasteners is provided by a fastener described in U.S. Pat. No. 2,562,376 to Cragg et al. That fastener includes a radially compressible nut disposed within a shell that compresses the nut onto a bolt as the nut is threadedly tightened in place. The radial compression is effected by a plurality of small protrusions projecting inwardly from the shell into engagement with the nut. During the initial threading of a bolt into the nut, the protrusions project into a circumferential groove formed in the nut's outer surface, whereas during the final threaded turn of the bolt, the groove will have moved axially away from the protrusions so that the protrusions compressively engage the nut's non-grooved outer surface. This radially compresses the nut onto the bolt and thereby locks it in place.
Although the fastener of the Cragg et al. patent is generally effective in locking itself onto a bolt only during the final threaded turn of the bolt, it is not believed to perform entirely satisfactorily. One drawback to the Cragg et al. fastener is that it is generally difficult to precisely control the sizes of the protrusions, which can lead to a variable compressive force. Another drawback is that the protrusions can become worn after repeated use, which can lead to a reduced compressive force.
It should, therefore, be appreciated that there still is a need for a self-locking threaded fastener that does not require application of a substantial torque throughout its entire range of threaded engagement with a bolt and that provides a predetermined, fixed self-locking force even after a significant number of repeated uses. The present invention fulfills this need.